How to Use the Day of the Week Calculator

One date in, one day of the week out — useful for checking a date far enough in the past or future that it's hard to picture on a calendar.

Any date, past or future, maps to exactly one day of the week — the Gregorian calendar repeats its weekday pattern with total regularity once you account for leap years correctly, which is exactly what this Day of the Week Calculator does using native date arithmetic rather than a lookup table. Enter a birthday, a historical date, or a future deadline and get back Monday through Sunday.